As such a machine tool, the gear hobbing machine disclosed in the Japanese Unexamined Patent Application Publication No. 2004-25333 is known, for example. This gear hobbing machine has a bed, a column provided on the bed in such a manner that it is movable in an X-axis direction which is a horizontal direction, a saddle supported on the front face of the column in such a manner that it is movable in a Z-axis direction which is the vertical direction, a hob head disposed on the saddle, a hob shaft which is supported by the hob head in such a manner that it is rotatable about its axis and is parallel to a horizontal direction (Y-axis direction) perpendicular to the X-axis direction and to which a hob is mounted coaxially, a hob-shaft driving motor for rotating the hob shaft about its axis via a gear train, a workpiece table which is provided on the bed in such a manner that it is horizontally rotatable and on which a workpiece is mounted coaxially, and a table driving motor for directly rotating the workpiece table horizontally.
In this gear hobbing machine, the rotational force of the hob-shaft driving motor is transmitted to the hob shaft via the gear train and thereby the hob shaft is rotated together with the hob, and the rotational force of the table driving motor is directly transmitted to the workpiece table and thereby the workpiece table is rotated together with a workpiece mounted thereon. At this time, the hob and the workpiece are rotated in such a manner that the phase of the hob and the phase of the workpiece correspond to each other. Thereafter, the column is moved in the X-axis direction to approach the workpiece and the saddle is moved in the Z-axis direction, and thereby the workpiece is machined by the hob.
The phase of the hob and the phase of the workpiece can be brought into correspondence with each other by, on the basis of, for example, detection signals obtained from a detection sensor for detecting that the rotational position of the hob-shaft driving motor is at its zero position and from a detection sensor for detecting that the rotational position of the table driving motor is at its zero position, bringing the rotational position of the hob-shaft driving motor and the rotational position of the table driving motor into correspondence with their respective zero positions and bringing the rotational position of the hob and the rotational position of the workpiece into correspondence with their respective zero positions, and then rotating the hob and the workpiece at their respective predetermined rotational speeds using their respective zero positions as reference. It is noted that the zero position means a rotational angular position which is a reference in the direction of rotation, and the zero positions of the hob-shaft driving motor and the hob are located at the same rotational angular position which is previously determined and similarly the zero positions of the table driving motor and the workpiece are also located at the same rotational angular position which is previously determined.